1. Field of the Invention
The present invention relates to a ocular position measuring apparatus and more particularly to instruments for examining patient's eyes for heterophoria and strabismus for example, phorometers or phoro-optimeters. More specifically, the present invention pertains to instruments having a pair of flickering marks emitting visible beams which are to be fixed upon by a patient.
2. Description of Prior Art
Abnormality of ocular position is generally classified into two categories, one being the heterophoria in which an eye deviates from the normal position with respect to the line of vision when the eyes are located in rest position and which can be eliminated through a binocular fusion, and the other being strabismus in which one of the eyes is unable to obtain binocular vision with the other because of an imbalance of the extraocular muscles.
In a conventional method for examining abnormality of the ocular position, a cross mark in located at a distance of for example 5 meters or more for examining a farsighted heterophoria and a small lamp or a toy is located at a distance of for example 33 centimeters for examining a near-sighted heterophoria. In both cases, a rectangular cardboard sheet of 15 centimeters by 25 centimeters is used as a blocking member which is to be inserted in front of a patient's eye while letting the patient watch the aforementioned marks. The movement of the pupil of the eye blocked by the blocking member is then inspected to determined whether there is a heterophoria.
In performing this inspection, it is desirable that the inspector sits in front of the patient in order to precisely inspect the movement of the pupil. It should however be noted that, since the inspector sitting in front of the patient may possibly block the patient's sight, the inspector is obliged to observe the patient's eyes from a front lower position. It should further be noted that in performing this type of inspection it is desirable that the eyes be at a complete rest, without any binocular fusion or accommodation when the blocking member is in front of an eye. Therefore, it is important to carry out the inspection in a completely dark room so that the patient cannot watch anything except the fixation mark. It should however be noted that the inspection must be actually made in a half dark room since the inspector must visually observe the movement of the patient's eye. Thus, there have been problems of inaccuracy of examination owing to binocular fusion and accommodation of the patient's eyes, and the difficulty of the observation of the patient's pupils in the half dark room.
As an alternative procedure for inspecting an abnormality of ocular position, there has been known an amblyoscope which is designed to make it possible for the inspector visually inspect movements of the patient's eyes. However, the amblyoscope has an eyepiece which has to be placed so that in front of the patient's eye, it is difficult to inspect the movement of the patient's eye.
It has also been known to use a phase difference haploscope for inspecting the abnormality of ocular position. The haploscope comprises a pair of rotating sectors which are adapted to be positioned in front of the patient's eyes and rotated at a high speed with 90.degree. phase difference with each other. A screen is provided for projecting visible images for both eyes through a pair of projecting sectors which are synchronized with the rotating sectors, so that the patient's eyes can catch only the images corresponding to the eyes, respectively. This type of inspecting instrument is however disadvantageous because the rotating sectors located in front of the patient's eyes make it difficult to inspect the patient's eyes.